Sansui AU-70 - Help Needed

[Dev0n]

Hi Everyone

I am new to diyAudio 🙂

I recently acquired an old Sansui AU 70 Tube amp.

I had a guy check it out, he told me all the old caps looked good and transformers tested ok.

He said he cleaned everything and replaced the 3 pre-amp tubes.

He did tell me that 2 of the 4 output tubes (7189) looked like they were overheating, but assured me that they are still working. Is this correct?

When i got the amp back a few weeks ago it was working perfectly.

This morning i turned it on and i heard a sound like when one used to press the degauz button on an old PC monitor.

A few seconds later i then heard a high pitched whistling sound coming from the amp, so i turned it off.

What could this be? The guy who fixed it said he isn't 100% sure what it could be, and assured me that transformers do make a "dull ringing sound" on start-up sometimes.

Does anyone have any advice as to what the sounds could mean?

I read online that it might be a failed tube, but i don't know.

Any help would be greatly appreciated. Thanks

 

[lexx21]

If those are original caps then they need to be replaced. Also if they were using the old carbon composite resistors, any that are out of tolerance need to be replaced as well.

You might want to take the amp to someone who is better familiar with tube amps and have them take a look at it. It sounds like the repair guy that you went to didn't realize a few fundamental things regarding vintage tube gear.

 

[Palustris]

I recently acquired an old Sansui AU 70 Tube amp. I had a guy check it out, he told me all the old caps looked good

You need to find a qualified technician. There is no way to tell if a capacitor is good or bad by looking at it and, more important, capacitors that are 50 years old absolutely need to be replaced. You need to find someone who understands this and who will completely rebuild the unit with new capacitors throughout. All out of specification resistors need to be replaced too and an entire new set of tubes would be an excellent idea. That Sansui is a high quality piece and when properly restored should easily compete with very costly modern equipment. I would definitely not power up the unit again until it is restored or you could damage a power or output transformer making the cost of restoration prohibitive.

 

[lexx21]

Yep... what I said....lol

 

[Palustris]

I was writing while you were posting and didn't see yours until after I posted. At least we agree!

 

[gingertube]

YES - Electrolytic Capacitors need replacing in any tube amp of this age.

AND A CAUTION

A lot of Japanese Tube Amps of this vintage had coupling caps that were labelled "Suzuki Oil". I don't believe that they really were oil caps but these caps had a bad habit of going leaky, and according to "Sods Law" usually the output tube grid coupling caps would leak and pull the bias way off such that the output tubes ran hot.
If your amp has any of these "Suzuki Oil" Caps in it, don't bother testing them, just rip them all out and replace them with modern polyproylene of the same value. Based upon my experience with them I can tell you 70% of them will be leaky so trying to identifying which are still Ok and which are not is not worth the effort. Just do the lot.
Cheers,
Ian

 

[Eli Duttman]

Ian is quite correct about the Suzuki caps. being trouble. Since voicing might be an issue, I suggest you use Soviet surplus K40 paper in oil (PIO) parts as the replacements. The Russian caps. are hermetically sealed and will not leak, either mechanically or electrically.

 

[directdriver]

Based upon my experience with them I can tell you 70% of them will be leaky so trying to identifying which are still Ok and which are not is not worth the effort. Just do the lot.

I agree completely. I own many Sansui tube receivers before and ALL of them had bad coupling caps. If they don't leak now, they will leak later. Yep, don't even bother salvaging them, just change them out so you will sleep better!


Here's the full schematic of the Sansui AU-70:

 

[Dev0n]

Hi Guys

Thanks for all the replies. I agree with everyone that everything will have to be checked and replaced. The problem is i can't find any professionals where I live (Cape Town, South Africa). I am new to "recapping" and this will be my first attempt. I had a look inside, and it looks like there might have been some work done.

Maybe one of you guys can have a look and tell me.

Thanks! 🙂